Open source JDK, that is. Sun Microsystems did it again -- As promised during JavaOne event back in May 2006, Sun made the implementation of the Java Platform, Standard Edition, available to the community under GNU General Public License (GPLv2).

Note that only HotSpot Virtual Machine and javac (compiler) components of the earlier builds of JDK version 7 are open sourced as of now. Rest of the components will be open sourced over the time; and by the end of first half of 2007, we will have fully buildable implementation for JDK 7.